Conventionally, as a control system of this kind, there has been known one disclosed e.g. in PTL 1 filed by the present applicant. In this control system, a fully-open intake air amount, which is an amount of intake air in a case where a throttle valve is in a fully-open state, is calculated according to a rotational speed of an internal combustion engine, and then a stoichiometric intake air amount, which is associated with an ideal state in which no exhaust gases are assumed to be recirculated into a combustion chamber, is calculated according to the fully-open intake air amount and an intake pressure. Then, an EGR ratio is calculated using the above stoichiometric intake air amount and an actual intake air amount of air actually drawn into the cylinder. By calculating the EGR ratio as described above, it becomes unnecessary to set in advance a large number of maps associated with various operating states of the engine, whereby the number of map setting steps can be largely reduced. The calculated EGR ratio is used for control of the engine, such as ignition timing control.